Systems and methods for implementing receiver transparent Q-mode

ABSTRACT

In a receiver transparent Q-mode, i.e., a Q-mode that is only implemented by a transmitter, the receiver is unaware of the Q-mode state of the transmitter. In this type of Q-mode configuration, the transmitter could enter and exit Q-mode as desired while the receiver, could, for example, continue to function as if operating normally, such as in “showtime.” Through this approach, it is not necessary for the receiver to detect the transmitter&#39;s entry and exit of Q-mode.

RELATED APPLICATION DATA

This application is a Continuation of U.S. patent application Ser. No.12/783,749, filed May 20, 2010, now U.S. Pat. No. 8,335,271, which is aContinuation of U.S. application Ser. No. 12/478,577, filed Jun. 4,2009, which is a Continuation of U.S. application Ser. No. 11/674,871,filed Feb. 14, 2007, now U.S. Pat. No. 7,558,329, which is aContinuation of U.S. application Ser. No. 11/434,249, Filed May 16,2006, which is a Continuation of U.S. application Ser. No. 11/200,002,filed Aug. 10, 2005, which is a Continuation of U.S. application Ser.No. 10/802,867, filed Mar. 18, 2004, which is a Continuation of U.S.application Ser. No. 10/106,329, filed Mar. 27, 2002, now U.S. Pat. No.6,731,695, which claims a benefit of and priority to U.S. ProvisionalApplication Ser. No. 60/278,936 filed Mar. 27, 2001, entitled “ReceiverTransparent Q-Mode,” U.S. Provisional Application Ser. No. 60/283,467filed Apr. 12, 2001, entitled “Receiver Transparent Q-Mode With On-LineReconfiguration,” U.S. Provisional Application Ser. No. 60/287,968 filedMay 1, 2001, entitled “Receiver Transparent Q-Mode With On-LineReconfiguration And Scrambling,” and U.S. Ser. No. 60/293,034 filed May23, 2001, entitled “Receiver Transparent Q-Mode With On-LineReconfiguration And Scrambling And Q-Mode Symbol Distortion,” each ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention general relates to multicarrier communication systems. Inparticular, this invention relates to systems and methods that allow atransmitter to enter and exit a Q-Mode.

2. Description of Related Art

Q-mode is being examined by ADSL standards bodies in the ITU-T in thedevelopment of the G.dmt.bis and G.lite.bis ADSL standards, both ofwhich are incorporated herein by reference in their entirety. Q-mode isa low power transmission mode intended to save power by transmittingsignals with lower PAR (Peak to Average power Ratio) with respect tonormal steady state, i.e., full power, signals. The Q-mode signal withlow PAR will often have the same average power as the normal steadystate signals but since the peak power is reduced, power consumption canbe reduced in the analog transmission circuitry. This is very importantespecially for saving power in telephone company central offices andremote cabinets where ADSL modems are often installed.

Current Q-mode proposals utilize a Q-mode “filler” symbol with low PARproperties in order to save power at the transmitter. Discussion of thistype of approach can be found in various ITU Documents, such as, BA-044,BA-045, HC-029R1, CF-033 and CF-040, all of which are incorporatedherein by reference in their entirety. Other ITU proposals state thatthe filler symbol should be defined by the transmitter and communicatedto the receiver during initialization, see BI-080 and D.87, both ofwhich are also incorporated herein by reference in their entirety.

SUMMARY OF THE INVENTION

However, current systems suffer from a number of drawbacks. Theseexemplary drawbacks include the necessity of receivers being required toimplement circuitry to detect Q-mode symbols on every DMT symbol.Furthermore, the entry and exit modes associated with the Q-mode lackrobustness since the receiver needs to be able to distinguish a Q-modesymbol from a real information-carrying symbol.

In contrast, the exemplary systems and methods of this invention focuson a receiver transparent Q-mode, i.e., a Q-mode that is onlyimplemented by the transmitter, wherein the receiver is unaware of theQ-mode state of the transmitter. In this type of Q-mode configuration,the transmitter could enter and exit Q-mode as desired while thereceiver, could, for example, continue to function as if operatingnormally, such as in “showtime.” Through this approach, it is notnecessary for the receiver to detect the entry and exit of Q-mode by thetransmitter.

Accordingly, exemplary aspects of the present invention relate tomulticarrier communications systems. In particular, an exemplary aspectof the invention relates to conserving power at a transmitter.

Additionally, aspect of the present invention relate to allowing atransmitter to enter into a Q-mode while the receiver is unaware of theoperational state of the transmitter.

Aspects of the present invention also relate to eliminating the need fora receiver to have Q-mode entry and exit detection capabilities.

Aspects of the present invention also relate to seamless changing intoand out of a Q-mode between a transmitter and a receiver.

Additional aspects of the present invention relate to sending aplurality of symbols from a transmitter to a receiver wherein thereceiver does not need to determine which of the symbols is a Q-modesymbol.

These and other features and advantages of this invention are describedin, or are apparent from, the following detailed description of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be described in detailed, withreference to the following figures, wherein:

FIG. 1 is a block diagram illustrating an exemplary portion of atransmitter according to this invention;

FIG. 2 illustrates an exemplary constellation point according to thisinvention;

FIG. 3 illustrates an exemplary 64-QAM constellation according to thisinvention;

FIG. 4 is a block diagram illustrating an exemplary XOR scrambleraccording to this invention;

FIG. 5 is a block diagram illustrating an exemplary transmitter havingan XOR scrambler and phase rotator for a 64-QAM constellation;

FIG. 6 is a block diagram illustrating an exemplary transmitter havingan XOR scrambler and phase rotator for use with trellis coding;

FIG. 7 is a flowchart illustrating an exemplary method of training thetransmitter and receiver according to this invention;

FIG. 8 is a flowchart illustrating an exemplary method of enteringQ-mode according to this invention; and

FIG. 9 is a flowchart illustrating an exemplary method of exiting Q-modeaccording to this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an exemplary transmitter according to an embodimentof this invention. In particular, the transmitter 10 comprises a serialto parallel converter 110, an XOR scrambler 120, a QAM (quadratureamplitude modulation) encoder 130, a phase rotator 140, an Inverse FastFourier Transform module 150, a sign inversion module 160 and apseudo-random bit sequence (PRBS) module 170, all interconnected bylinks 5.

The exemplary systems and methods of this invention will be described inrelation to a multicarrier modulation communication system. However, toavoid unnecessarily obscuring the present invention, the followingdescription omits well-known structures and devices that may be shown inblock diagram form or otherwise summarized. For the purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the present invention. It should beappreciated however that the present invention may be practiced in avariety of ways beyond these specific details.

While the exemplary embodiments illustrated herein show the variouscomponents of the transmitter, and corresponding receiver, collocated,it is to be appreciated that the various components of the system can belocated a distant portions of a distributed network, such as atelecommunications network and/or the Internet or within a dedicatedcommunications network. Thus, it should be appreciated that thecomponents of the transmitter and receiver, respectively, can becombined into one or more devices or collocated on a particular note ofa distributed network, such as a telecommunications network. As will beappreciated from the following description, and for reasons ofcomputational efficiency, the components of the communications networkcan be arranged at any location within the distributed network withoutaffecting the operation of the system.

Furthermore, it should be appreciated that the various links connectingthe elements can be wired or wireless links, or a combination thereof,or any known or later developed element(s) that is capable of supplyingand/or communicating data to and from the connected elements.

As used herein, the following nomenclature is represented by thefollowing symbols wherein:

-   -   N total number of carriers used for transmission    -   C_(i) the i^(th) carrier, where I=0 . . . N    -   B_(i) the number of bits modulated by C_(i) (as defined by the        receiver during training)    -   G_(i) the fine gain of C_(i) (as defined by the receiver during        training)    -   Q_(i) the 4-QAM constellation point of C_(i) of the Q-mode        symbol (as defined by the transmitter)    -   w_(i) the input bit word to be modulated on C, (with the length        of this word being B_(i) bits)    -   s_(i) the XOR scrambled w,    -   q_(i) the QAM symbol of C,    -   X_(i) the XOR scrambler bit values for C, and    -   R_(i) the phase rotation value of C_(i)

In operation, L bits are received from a transmission protocolspecific-transmission convergence layer (not shown) and converted, withthe cooperation of the serial to parallel converter 110, into w_(N)words. These w_(N) words are then processed by the XOR scrambler 120 andphase rotator 140, which work in cooperation, to map an “all zeros”input word (w_(i)) of the i^(th) carrier to a point in a constellationdefined by B_(i),G_(i) that is closest to the 4-QAM Q-mode constellationpoint (Q_(I)) for that carrier. At the receiver, not illustrated, theinverse operations are performed.

As discussed above, an exemplary purpose of the XOR scrambler 120 andthe phase rotator 140 is to map the “all zeros” input word (w_(i)) tothe constellation point that is closes to the Q-mode constellation point(Q_(i)) for each carrier. As a result, a different PMD scrambler andphase rotation is defined for each carrier. The XOR scrambler 120 andphase rotator 140 depend on, for example, the QAM constellation size(B_(i)) of the carrier, the fine gain (G_(i)) of the carrier and theQ-mode 4-QAM constellation point (Q_(i)) of the carrier which can be,for example, defined by the transmitter or receiver.

As illustrated in FIG. 1, the XOR scrambler 120 and phase rotator 140operate on a carrier by carrier bases. Thus, as illustrated in exemplaryFIG. 2, and assuming that the carrier has been specified by thetransmitter to utilize Q_(i)={+,−} or {0, 1} for the Q-mode symbol, theXOR scrambler bits (X_(i)) and the phase rotation value (R_(i)) of acarrier that has B_(i)=6 bits and a fine gain of G_(i)=√(21/13)=1.27(2.08 db). After scaling by the factor G, FIG. 3 illustrates the 64-QAMconstellation, wherein G=G_(i)*(constellationgain)=√(21/13)(1/√(21))=√(1/13), wherein 1/√(21) is the constellationgain scaling for a 64-QAM constellation relative to the 4-QAMconstellation. Based on this scaled 64-QAM constellation, theconstellation point in the lower right hand quadrant that is closest tothe {+,−} constellation point of the Q-mode signal in FIG. 1 can bedetermined. A pure phase rotation of the entire constellation allows theuse of any 64-QAM constellation point, i.e., not only those on thediagonal. Therefore, the 64-QAM constellation point that has a distanceof √(2) from the origin need to be determined since this is the distanceof the Q_(i)={+,−} Q-mode point. The distance from the origin of the64-QAM constellation points in the lower right hand quadrant are:D(1,1)=G*√(1+1)=√(1/13)*√(2)D(1,3)=D(3,1)=G*√(1+9)=√(1/13)*√(10)D(1,5)=D(5,1)+G*√(26)=√(1/13)*√(26)=√(2)D(3,3)=G*√(9+9)D(3,5)+D(5,3)=G*√(9+25)D(5,5)=G*√(25+25)D(1,7)=D(7,1)=G*√(1+49)D(3,7)=D(7,3)=G*√(9+49)D(5,7)=D(7,5)=G*√(25+49)D(7,7)=G*√(49+49)

From the calculation it is apparent that the point at (1,5) has adistance of √(2) from the origin and is chosen as the constellationpoint to be used for the “all zeros” input word on this exemplaryconstellation. The (1,−5) constellation point is mapped to the bitpattern {010101} in the 64-QAM constellation. Therefore, the scramblerfor this constellation point will map the all-zeros word {000000} to{010101}. A simple scrambler that can accomplish this is an XOR of theinput word for this constellation with the bit pattern X_(i)={010101}.

FIG. 4 illustrates an exemplary XOR scrambler that is capable ofperforming the scrambling set forth in the above example. Specifically,the XOR scrambler receives an input word to be modulated that comprisesall zeros, XORs that with the XOR scrambler bit values for C_(i) andoutputs the appropriately scrambled XOR scrambled input word s_(i). Asan example, FIG. 4 shows the XOR scrambler for a carrier using theQ-mode constellation shown in FIG. 2 and the 64-QAM constellation shownin FIG. 3 using the fine gain of 2.08 dB.

In addition to the XOR scrambler illustrated in FIG. 4, the phaserotation value R, needs to be specified in order to migrate the (1,−5)constellation point to the diagonal. A phase rotation of R=π/5.34 willplace the (1,−5) point on the diagonal 64-QAM constellation. Of course,this phase rotation will be applied to the entire 64-QAM constellationthus maintaining the same distance properties and average power for theentirety of the constellation.

As illustrated in FIG. 5, the phase rotation is applied by the phaserotator 140 after the QAM encoding is performed by, for example, the64-QAM encoder 130. The result is then passed through the IFFT module150 and output.

As discussed above, each carrier will have a different XOR scrambler andphase rotation. The parameters for the XOR scrambler 120 and the phaserotation for the phase rotator 140 for each carrier depend on, forexample, the QAM constellation size for the particular carrier, the finegain value used on that carrier and the Q-mode 4-QAM constellation pointof that carrier.

Even though not illustrated, at the receiver, the inverse functions tothat of the transmitter are performed. Specifically, the received signalis rotated back at the output of an FFT module and descrambled at theoutput of the QAM decoder to obtain the original input word w_(i).

FIG. 6 illustrates an example of XOR scrambling and phase rotation usingtrellis coding. In particular, where trellis coding is used, the XORscrambler 120 scrambles only the uncoded bits. The coded bits will passthrough the convolution encoder 125 unscrambled. This results in anall-zeros word always being mapped to a constellation point in coset 0.If the constellation point used to map the all-zeros input word to the4-QAM Q-mode constellation pint is not in coset 0, then the entireconstellation can be rotated to produce the correct results.

As an illustrative example, assume trellis coding is used for the aboveexample. In the above example, the chosen constellation point (1,−5) islocated in coset 1. However, in the (+,+) quadrant the constellationpoint (5,1) is located in coset 0. Therefore, rotating the entireconstellation by −π/2 will move the (5,1) constellation point to the(1,−5) position and, as a result, the rotated (1,−5) constellation pointwill be located in coset 0.

FIG. 6 illustrates this encoding process. In particular, the bit valuesmapped to the unrotated (5,1) constellation point are {010000}. In thiscase, the XOR scrambler value is X_(i)={0100} is applied by the XORscrambler 120 to only to the uncoded bits (4MSBs). The coded bits(2LSBs) that designate the coset will be encoded with the convolutionencoder 125 without scrambling. The resulting QAM symbol is located incoset 0 at constellation point (5,1). A phase rotation of −π/2 is thusrequired to move this QAM symbol to the (1,−5) position. Thus the −π/2phase rotation is added to the phase rotation R_(i)=π/5.34−π/2 by thephase rotator 140.

Alternatively, for example, the XOR mapping is applied to all bits, evenwhen trellis coding is enabled. In this case, however, an additionalstep is required in the receiver's trellis decoder. Normally, the firststep in trellis decoding is to determine the four 2-D metrics for eachpair of constellation points. These 2-D metrics are then combined toform eight 4-D metrics. If the coset bits are XORed at the transmitter,then this XORing be accounted for in the determination of the 4-Dmetrics prior to the normal decoding. A simple way to do accomplish thisis to modify the 4-D metric computation table with the appropriate XORvalues.

For example, let C₂ ^(i) be the i^(th) 2D coset and let C₄ ^(i) be thei^(th) 4D coset. By definition C₄ ⁰=min {C₂ ⁰+C₂ ⁰,C₂ ³+C₂ ³}. Now, letH₁ be the 2 bit XOR pattern for the first 2D tone, and let H₂ be the 2bit XOR pattern for the second 2D tone. The 4D metric computation isthus:

C₄⁰ = min {C₂^(0 ⊕ H₁) + C₂^(0 ⊕ H₂), C₂^(3 ⊕ H₁) + C₂^(3 ⊕ H₂)}

Therefore, the XOR operation can be removed at the receiver during thedetermination of the 4D metrics. Note that the 2 bit pattern is simplythe first 2 bits of the XOR pattern implemented at the transmitter, andthe higher level XOR bits can be removed after the trellis decoding.

The transmitter and receiver can also determine a loss in margin, ifany, when the transmitter sends the Q-mode symbol based on a differencebetween the Q-mode symbol and the scrambled all-zeros symbol. This lossand margin occurs, for example, if the receiver does not map theall-zero symbol exactly to the Q-mode symbol. While it can be acceptableto have a loss in margin during transmission of the Q-mode symbol sincethe Q-mode symbol contains no information bits, in the case of a largemargin loss, the transmitter and receiver must account for, for example,possible false alarms such as CRC errors, FEC errors, HEC errors, andthe like, during the Q-mode symbol transmission. Otherwise datatransmission is unaffected.

As discussed above, a loss in margin occurs, for example, if thereceiver does not map the all-zero symbol exactly to the Q-mode symbol.Alternatively, a loss in margin can be reduced by using the fine gains.In particular, the receiver transparent Q-mode places restrictions onthe values of the fine gains that may be used if it is desired to have areceiver without any loss in margin during the reception of the Q-modesymbol. For large constellations, such as, for example, B_(i)≧6 bits,there are several possible fine gain values that will align a point inthe constellation with the Q-mode constellation point. For example, forsmaller constellations, such as B_(i)≦6, there are fewer allowable finegain values.

A number of exemplary issues can determine the number of fine gainvalues. In particular, the fine gain can be chosen such that either theexact Q-mode constellation point is used, or the fine gain is such thatthe actual Q-mode constellation point can be transmitted without a lossof margin even though the transmitted point is not the point that wouldhave be chosen during a showtime transmission. In other words, incertain cases, the constellation point can be forced to the value thatis needed for Q-mode without a loss of margin at the detector. Thisoccurs when an all-zero input is mapped to the outermost constellationpoint along one of the 45° diagonals and the Q-mode symbol chosen to liefurther along this diagonal.

Table 1 illustrates exemplary allowable fine-gain values (in dB) if areceiver operates with zero margin loss during Q-mode transmission.

TABLE 1 B_(i) = 1 B_(i) = 2 B_(i) = 3 B_(i) = 4 B_(i) = 5 B_(i) = 6B_(i) = 7 B_(i) = 8 <0 <0 −2.2 <−2.55 −2.3 <−3.7 −3.1 <−4.2 0 −1.1 −2.4−2.5 −3.6 0.45 −1.4 −2.0 −3.1 3.0 −0.75 −1.7 −2.0 0.92 −1.1 −2.6 2.08−0.40 −2.4 0 −2.3 0.44 −2.1 1.5 −1.7 2.1 −1.7 3.8 −1.5 −1.4 −1.3 −1.2−1.1 −0.75 −0.57 −0.20 0 0.21 0.66 1.2 1.4 2.0 2.4 2.8

With reference to Table 1, note that the exemplary fine gain values arevalid for a Q-mode symbol with no fine gain, i.e., g_(sync)=0 dB.

For B_(i)=1 and B_(i)=2, the receiver may use any negative fine gainvalue since the Q-mode constellation point can be mapped to a point onthe diagonal and therefore the Q-mode symbol will not effect the biterror rate (BER) performance and, in actuality, will increase theminimum distance properties of the constellation during the Q-modetransmission.

The same principles can be applied to all even constellations, i.e., thenegative fine gains that result in the Q-mode constellation point beingmapped to a point that is further from the origin than the outermostconstellation point of the B_(i) constellation do not result in a lossin margin during transmission of the Q-mode symbol. However,

Exemplary advantages of the above-described receiver-transparent Q-modeinclude design flexibility for a receiver. In particular, there are aplurality of exemplary trade-offs that a receiver my make inimplementing the exemplary systems and methods of this invention.

First, there is a data rate trade-off where a receiver may chose to useonly the fine gain values in Table 1 if the receiver wants to assureloss in margin during reception of the Q-mode symbol. In doing so, theremay be a small loss in data rate. However, this data rate loss may be anacceptable trade-off for a receiver that does not implement thecomplexities of detecting the entry and exit Q-mode symbols. Forexample, bit loading algorithms may be used to avoid a loss in datarate.

Alternatively, a receiver may chose not to use the fine gain values inTable 1 and still operate in a receiver-transparent Q-mode. As a result,there can be an effective decrease in margin during the reception of theQ-mode symbol. However, this decrease in margin during the Q-mode symbolwill not effect the true information carrying symbols and therefore maybe an acceptable trade-off for a receiver that does not wish toimplement the complexities of detecting the Q-mode entry and exitsymbols.

Furthermore, a receiver may chose to not use the fine gain values inTable 1 and not operate in a receiver-transparent Q-mode. In this casethe receiver may chose to operate as in the current Q-mode proposals inG.dmt.bis and implement the complexities of detecting the entry and exitQ-mode functions. The receiver could notify the transmitter that it isnot using the XOR and phase rotation functions and this could beaccomplished by sending, for example, a message from the receiver to thetransmitter indicating this type of operation. Alternatively, thereceiver could send to the transmitter an X, table with all-zeros, suchthat the all-zeros input words are mapped to the true all-zerosconstellation point.

Thus, one exemplary benefit of the systems and methods of this inventionis that it allows the receiver to decide what method the receiver willbe using for Q-mode. Therefore, the complexity burden to implement theQ-mode can be, for example, determined by the transmitter which, if thetransmitter opts to implement Q-mode, the transmitter will implement theXOR scrambler and phase rotation even if the receiver decides to not usethose functions, i.e., the receiver chooses to detect the Q-mode fillersignal.

In the current Q-mode proposals in G.dmt, the Q-mode signal israndomized by alternating the reverb/segue signals based on a PRBS withperiods longer than 4,000 symbols. The same method of randomization canbe used in conjunction with the systems and methods of this invention.However, in this case, all DMT symbols, both information-carrying DMTsymbols and the Q-mode symbol, are inverted based on the PRBS. This canbe implemented by, for example, alternating the signs of all DMT symbolsat the IFFT output based on the PRBS. Alternatively, the phase shift maybe implemented as part of the phase rotation in the phase rotator 140.This will allow, for example, phase shifts other than just 180°, i.e.,simple inversion. For example, the DMT symbols could be furtherrandomized based on the 90° phase shifts.

Multicarrier ADSL systems typically use a synchronous bit scramblerbefore modulation in order to assure that the data bits beingtransmitted are as random as possible, this is important to keep thePeak to Average Power ratio low in multicarrier modems. The XORscrambler 120 as discussed herein however, does not provide thisrandomization function since it is primarily just mapping bits from onepattern to another. In order to support the exemplary bit scramblingfunctions according to this invention, one of the following exemplaryembodiments can be implemented:

First, a self-synchronizing scrambler can be placed before the XORscrambler at the transmitter. If FEC coding is used, then theself-synchronizing scrambler would typically be placed before the FECcoder at the transmitter. The self-synchronizing scrambler is then resetafter scrambling S bits. Upon the scrambler being reset, the initialscrambler state is reset to all zeros. Also, upon the scrambler reset,the scrambler feedback connections may be changed. For example, thescrambler feedback may be changed based on a different pseudo-randompattern. For example assume the scrambler was defined as:D _(n) ′=D _(n) ⊕D _(n-17) ′⊕D _(n-25)′,

where D_(n) is the n^(th) data bit at the input of the scrambler andD_(n)′ is the n^(th) data bit at the scrambler output. After S bits areinput to the scrambler, the scrambler is reset to the all zero state andthe feedback connections changed to, for example, n−3 and n−20. Thesenew connections can be updated, for example, based upon a secondscrambler, and the number of connections need not be fixed. The value ofS in the second scrambler, or for example a random number generator,that defines the new delay values would be known by the transmitter andthe receiver so that the receiver could reverse the scramblingoperation, i.e., descramble, upon scrambler reset.

Another method which does not involve changing the scrambler connectionsis based on defining S in such a way as to assure that a repetitive bitpattern at the input of the scrambler will not result in the same bitpattern being modulated on the same carriers in a plurality of DMTsymbols. For example, this can be accomplished by defining S and L,where L is the number of bits modulated on all the carriers in singleDMT symbol, to have no common factors other than 1, i.e., S and L aremutually co-prime.

Second, the data bits can be interleaved (shuffled) at some point priorto the XOR scrambler. This interleaving pattern can change from one DMTsymbol to the next. The interleaving pattern could be known by thetransmitter and receiver so that the operations can be reversed by thereceiver. For example, assume that 1 is the number of bits modulated onall carriers in a single DMT symbol. A simple interleaver could move Nbits to the end of the length L bit stream on the N^(th) DMT symbol. Forexample if [B1,B2,B3,B4 . . . BL] was the bit pattern at the input ofthe interleaver on the 3rd DMT symbol, then [B4,B5,B6, . . .BL,B1,B2,B3] would be the bit pattern at the interleaver output.However, it should be appreciated that any other interleaving patterncan be used as long as it is known by the transmitter and the receiver.

As an alternative to the above exemplary embodiments of the systems andmethods of this invention, instead of having the receiver define theB_(i) and G_(i) tables such that the Q-mode symbol is mapped to theall-zero symbol, the Q-mode symbol may be defined by the transmitter andbe restricted to valid constellation points. The resulting Q-mode symbolmay not have the exact PAR characteristics desired by the transmitter,but this Q-mode symbol will also map exactly to the all-zero symbol.

Constellation points chosen for the Q-mode symbol may be determined bythe transmitter and communicated to the receiver after the B_(i) andG_(i) tables have been communicated. Alternatively, the constellationpoints chosen for the Q-mode symbol may be determined independently bythe receiver and the transmitter based on the desired Q-mode symbol,such as a Q-mode symbol sent from the transmitter to the receiver duringtraining.

Alternatively, tone reordering is another method that can be use duringbit loading to achieve better mapping of the all-zero symbol to theQ-mode symbol. This can be helpful when trellis coding or some othercode, such as turbo code, is used.

Alternatively still, the Q-mode signal may be also be predefined in astandard, or, for example, by a system provider, as opposed to bedefined by the transmitter.

Furthermore, instead of exchanging the X_(i) table, the transmitter andreceiver may independently determine the X_(i) values by finding theconstellation points that are closest to the Q-mode constellation point.From this value, the transmitter and receiver can independently generatethe XOR scrambler and the phase rotation values required. In order todetermine the X_(i) values independently, there should be a set of rulesdefined to ensure that the same constellation points are chosen by boththe transmitter and receiver. This is important when two or moreconstellation points satisfy the condition of being closest in magnitudeto the Q-mode point. For example, in the disclosed example, theconstellation points (1,−5) and (5,−1) are the √2 distance from theorigin. In this case, a rule such as “always pick the point that resultsin the smallest positive phase rotation value” can be chosen and thiswould ensure that both the transmitter and receiver would be pick theconstellation point (1,−5). Alternatively, tables can be stored at oneor more of the transmitter and receiver and values chosen from thosetables for X_(i) values.

In the case where a synchronized symbol is being transmitted every 69DMT symbols, as is done in the G.992.1 and G.992.2 standards, a systemwith a receiver transparent Q-mode operating in accordance with thesystems and methods of this invention can send a Q-mode symbol as a SYNCsymbol. This way, when the transmitter enters the Q-mode, the SYNCsymbol could continue to be sent unaltered.

Still further, on-line reconfiguration is possible such that whenperforming changes to the B_(i) and G_(i) tables during showtime, e.g.,bit swapping on-line reconfiguration, in addition to the new B_(i) andG_(i) values, the receiver could also send to the transmitter the newcorresponding X_(i) values. If the receiver and the transmitter areindependently determining the X_(i) values, then it is not necessary forthe receiver to send the new X_(i) values during a showtime change ofthe B_(i) and G_(i) tables.

The ADSL framing parameters can also be configured in such a way as toensure that non-scrambled data sequences will be mapped to differentcarriers from one DMT symbol to the next. As an example, ATM cellheaders are not scrambled by the ATM TPS-TC. If the number of bits in aDMT symbol is chosen to be mutually co-prime with respect to the MUXdata frame size in the ADSL modems, then when the SYNC bits are insertedinto the data bit stream, the ATM cell headers will be mapped todifferent carriers from one DMT symbol to the next. Alternatively, ifthe number of bits in a DMT symbol is chosen to be mutually co-prime,with respect to the ATM cell size, such as 53 bits, then the ATM cellheaders will be mapped to different carriers from one DMT symbol to thenext. Alternatively still, if the number of bits in a DMT symbol ischosen to be mutually co-prime with respect to the Reed-Solomon codeword size, e.g., 53 bits, then the ATM cell headers will be mapped todifferent carriers from one DMT symbol to the next. Obviously,configuring other framing parameters in this manner will have the sameresults.

FIG. 7 outlines an exemplary method of training a transmitter and areceiver according to this invention. In particular, control begins instep S100 and continues to S110. In step S110, the transmitter specifieson all carriers the Q-mode filler symbol as any pseudo-random 4-QAM DMTsymbol. During this training, the transmitter sends the bit pattern(Q_(i)) for the Q-mode symbol to the receiver before the B_(i) and G_(i)tables are determined. Next, in step S120, the receiver determines theB_(i) and G_(i) tables. Alternatively, the receiver may take intoaccount the Q-mode symbol characteristics during bit loading in order toensure that a scrambled all-zero symbol is as similar as possible to thetransmitter defined Q-mode symbol. As described above, this can beaccomplished by mapping the all-zeros input word to a constellationpoint that is chosen to be as close as possible, or even identical, tothe Q-mode constellation point for the carrier. Thus, in step S130, adetermination is made whether the receiver will take into account theQ-mode symbol characteristics during bit loading. If the receiver does,control continues to step S140 where an all-zeros input word is mappedto a predetermined constellation point. Otherwise, control jumps to stepS150.

In step S150, the receiver sends the B_(i), G_(i) and X_(i) tables whichcontain the bit patterns of the chosen constellation points for eachcarrier that map the all-zeros input word (w_(i)) to the Q-modeconstellation point (Q_(i)). Then, in step S160, based on the chosenconstellation point, both the transmitter and receiver generate the XORscrambler and phase rotation for each carrier. Control then continues tostep S170 where the control sequence ends.

FIG. 8 outlines an exemplary method of entering Q-mode according to thisinvention. In particular, control begins in step S200 and continues tostep S210. In step S210, an all zero bit pattern is received by thetransmitter. Next, in step S220 a determination is made whether theQ-mode symbol will be sent to the receiver. If the Q-mode symbol is tobe sent to the receiver, thus allowing the transmitter to enter Q-mode,control continues to step S250. Otherwise, control jumps to step S230.

In step S250, the Q-mode symbol is forwarded to the receiver. Next, instep S260, the transmitter enters Q-mode. Then, in step S270, thereceiver demodulates the Q-mode symbol and continues processing as ifthe receiver was operating during normal showtime. Control thencontinues to step S280 where the control sequence ends.

In step S230, the transmitter transmits the modulated all-zero symbol.Then, in step S240, a determination is made whether the modulatedall-zero symbol is the same as the Q-mode symbol. If the modulatedall-zero symbol is the same as the Q-mode symbol control continues tostep S260 where the transmitter enters Q-mode. Otherwise, controlcontinues to step S280 where the control sequence ends.

FIG. 9 outlines an exemplary method of exiting Q-mode according to thisinvention. In particular, control begins in step S300 and continues tostep S310. In step S310, a determination is made whether actualinformation bits are to be transmitted. If actual information bits areto be transmitted, control continues to step S330. Otherwise, controlcontinues to step S320 where the transmitter remains Q-mode. Controlthen returns back to step S310.

In step S330, the transmitter will exit Q-mode by sending DMT symbolsmodulated with the real information bits. Next, in step S340, thereceiver will receive and seamless demodulate the transmitted actualinformation bits. Control then continues to step S350 where the controlsequence ends. Thus, the receiver will have transitioned through thetransmitter cycling into and out of Q-mode without the receiver needingto detect this transition nor detect the exit of the transmitter fromQ-mode.

The present invention for a receiver transparent Q-mode in amulticarrier transmission system can be implemented either on a DSLmodem, such as an ADSL modem, or separate programmed general purposecomputer having a communication device. The present method can also beimplemented in a special purpose computer, a programmed microprocessoror a microcontroller and peripheral integrated circuit element, an ASICor other integrated circuit, a digital signal processor, a hardwired orelectronic logic circuit such as a discrete element circuit, aprogrammable logic device, such as a PLD, PLA, FPGA, PAL, or the like,and associated communications equipment.

Furthermore, the disclosed method may be readily implemented in softwareusing object or object-oriented software development environments thatprovide portable source code that can be used on a variety of computers,workstations, or modem hardware and/or software platforms.Alternatively, the method may be implemented partially or fully inhardware using standard logic circuits or a VLSI design. Other softwareor hardware can be used to implement the methods in accordance with thisinvention depending on the speed and/or efficiency requirements of thesystem, the particular function, and the particular software and/orhardware or microprocessor or microcomputer(s) being utilized. Ofcourse, the present method can also be readily implemented in hardwareand/or software using any known later developed systems or structures,devices and/or software by those of ordinary skill in the applicable artfrom the functional description provided herein and with a general basicknowledge of the computer and telecommunications arts.

Moreover, the disclosed methods can be readily implemented as softwareexecuted on a programmed general purpose computer, a special purposecomputer, a microprocessor and associated communications equipment, amodem, such as a DSL modem, or the like. In these instances, the methodsand systems of this invention can be implemented as a program embeddedin a modem, such as a DSL modem, or the like. The methods can also beimplemented by physically incorporating operation equivalents of themethods into software and/or hardware, such as a hardware and softwaresystem of a multicarrier information transceiver, such as an ADSL modem,VDSL modem, network interface card, or the like.

While this invention has been described in conjunction with a number ofembodiments, it is evident that many alternatives, modifications andvariations would be or are apparent to those of ordinary skill in theapplicable art. Accordingly, applicants intend to embrace all suchalternatives, modifications, equivalents and variations that are withinthe spirit and the scope of this invention.

We claim:
 1. A method for a multicarrier transmitter, comprising aplurality of carriers, to enter a Q-mode, the method comprising:receiving a plurality of bits; scrambling a plurality of words using anXOR scrambler, wherein each word comprises one or more bits of theplurality of bits; mapping all or part of each scrambled word to a QAMconstellation to generate a plurality of QAM constellation points;modulating each carrier of the plurality of carriers of a Q-mode symbolusing one of the plurality of QAM constellation points; transmitting theQ-mode symbol; and entering the Q-mode, wherein the entry of the Q-modeis independent of an operational state of a receiver.
 2. The method ofclaim 1, further comprising determining one or more fine gain values forone or more of the plurality of carriers and a number of bits modulatedon each of the plurality of carriers.
 3. The method of claim 1, furthercomprising mapping an all zeros word of the Q-mode symbol to apredetermined constellation point.
 4. A multicarrier transmittercomprising: an XOR scrambler configured to scramble a plurality ofwords, wherein each word comprises one or more bits of a plurality ofbits; a QAM encoder configured to map all or part of each scrambled wordto a QAM constellation to generate a plurality of QAM constellationpoints; a modulator configured to modulate each carrier of a pluralityof carriers of a Q-mode symbol using one of the plurality of QAMconstellation points; and a transmitter configured to transmit theQ-mode symbol and enter a Q-mode, wherein the entry of the Q-mode isindependent of an operational state of a receiver.
 5. The multicarriertransmitter of claim 4, wherein the modulator is further configured todetermine one or more fine gain values for one or more of the pluralityof carriers and a number of bits modulated on each of the plurality ofcarriers.
 6. The multicarrier transmitter of claim 4, wherein the XORscrambler and a phase rotator are configured to map an all zeros word ofthe Q-mode symbol to a predetermined constellation point.